Process of refining metals



Z,8i?,585 Patented Dec. 24, 1957 due PROCESS on nnrnsnso METALS Charles H. Winter, In, Wilmington, DeL, assignor to E. I. (in Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application October 23, 1953 Serial No. 388,074

4 Claims. (Cl. 7584.1)

This invention relates to the preparation of group IV-A metals and especially pure, ductile titanium metal, by reducing a titanium halide, such as the chloride, with a metallic reducing agent, such as magnesium. More particularly, it relates to the purification and melting of crude titanium sponge produced by such metal reduction methods.

This application is a continuation-in-part of my copending application Ser. No. 118,176, filed September 27, 1949 (now abandoned).

While various other methods for the production of pure titanium metal have been proposed, the method which has been adopted commercially comprises the reduction of a vaporized halide of titanium by a metal such as magnesium in a reactor maintained at temperatures ranging from about 800-l000 C. and in accordance with, for instance, the disclosures of U. S. Patents 2,148,345 and 2,205,854. The processes of these patents produce titanium in sponge form and in association with the halide (having an atomic number greater than 9) of the metal used for the reduction. Commercial operations require the separation of these two reaction products, and various techniques have been proposed. A portion of the halide, e. g., magnesium chloride, may be drained as a molten salt from the sponge but a substantial percentage remains therein. The use of vacuum at a temperature around 1000 C. or higher has also been proposed as a method of removing magnesium chloride. This gives a purified sponge of excellent quality and a product superior to that which is obtained When one resorts to removal of the chloride by water-leaching methods. Such operations as these are not conducive to low cost of production of titanium in ingot or massive form, as considerable expense is involved in effecting the separation, and one still has to convert the sponge into massive form by a melting operation.

As indicated above, one is able to distill metal halides from titanium metal and without reversal of the reaction which produced the sponge titanium. The alkali and alkaline earth metal halides resulting from sponge production, e. g., MgCl NaCl, etc., boil at a temperature of around 1400-1500 C. and below the melting point of titanium metal. Purification methods wherein the halide is volatilized have been conducted under vacuum and considerably below the boiling point of the halide. The result has been satisfactory from a quality viewpoint but has left much to be desired in the way or" operating cost.

It is among the objects of this invention to overcome the above and other shortcomings of the prior art. A specific object is to separate titanium metal from the halide salt of the metal used in the conversion of a halide of titanium into the element. Another object is to prepare titanium in ingot form directly from titanium sponge in admixture with a metal halide salt. A still further object is a simplification of methods now used in the production of titanium metal, wherein crude titanium sponge is separated from contaminating metal halide while the two materials are in liquid condition. Other objects will become apparent from the detailed description of my invention.

These objects are obtained by my process which broadly comprises subjecting sponge titanium, in admixture with the halide of a more active metal which had been used in the production of said sponge titanium, to a temperature in excess of about 1725 C. or the melting point of the titanium, while maintaining a pressure at least equal to the vapor pressure of the said halide salt. Such conditions will cause the feed material to liquefy, molten titanium sinking to the bottom While the halide salt floats thereon. The pressure is sufficient to prevent vaporization of the halide salt, and thereby loss of heat through vaporization of the said salt is avoided, and thereafter the pure titanium is recovered.

A specific and preferred embodiment of my invention comprises the separation of titanium metal from a product such as is obtained from the process as outlined in U. S. Patent 2,205,854. The titanium sponge from such process is intimately associated with magnesium chloride, and this composite material is fed into a titanium-lined, externally cooled pressure vessel maintained at a temperature slightly above 1725 C. (the normal melting point of titanium) and at a pressure of from 5-10 atmospheres. The magnesium chloride of the feed material melts upon reaching the temperature of 708 C. and, if such has not been done previously, a substantial portion of this molten salt is removed without adding further heat thereto. Upon reaching the temperature of 1725 C., the titanium metal becomes liquid and two layers immediately appear in the melting furnace. The titanium metal then can be recovered from the melting furnace in either liquid or solid condition, as desired. Thus, the magnesium chloride can be drawn from the top and titanium metal may be removed from the bottom of such furnace through suitable withdrawal outlets or by the use of other wellknown methods. An attractively useful method is to eifect removal of the molten titanium under controlled cooling, so that it is removed as formed in solid rod or other massive condition. The cooling should be such that a portion of the titanium in the hot zone remains liquid at all times (assuming continuous operation is being carried out) so that a continuous stick or ingot of titanium is formed for withdrawal.

It is the combination with melting of such high-pressure techniques that renders my invention so effective. It is critical to my process that the halide by-product remain liquid; it is thus obvious that the pressure employed should at least equal the vapor pressure of said halide under the particular melting conditions obtaining in the vessel. Since vapor pressure varies with temperature, naturally the pressure within the vessel must be dependent upon the temperature utilized to efiect melting of the titanium. The melting point of titanium metal is about 1725 C. under normal conditions, and it is usually suilicient and preferred to maintain the melting furnace at such temperature, although a higher point may be reached if desired. As mentioned, the minimum pressure should be at least equivalent to the vapor pressure which the halide salt would have at 1725 C.; to name a few examples, for magnesium chloride this would be about pounds per square inch absolute pressure; for sodium chloride, 73 p. s. i. a.; and for lithium chloride, p. s. i. a. Maximum pressure is limited practically only by the problem of equipment and materials I of construction. Extremely high pressure conditions are generally too costly to utilize when not absolutely necesa (-1 sary; and such are not necessary to my process. It may be said that pressures of from 1 to 6 times the vapor pressure of the reducing metal halide at the melting temperature are usually satisfactory, and that a range of pressures which are from 1 to 10 times said vapor pressure is preferred and most commercially feasible. In the case of magnesium chloride, as an example, this would mean a broad range of75 to 450 p. s. i. a., and preferred limits of from 90 to 210 p. s. i. a. Similar conditions may easily be ascertained for the other common reducing metals and their halide by-products.

Specific equipment and techniques for melting the crude sponge and maintaining the inelting'furnace or vessel under pressure'may be any such as are known in the engineering art. An atmosphere of an inert gas such as hydrogen or argon or other noble gas is generally introduced to purge the system of air (which would react with the titanium metal). Such gas also may serve to maintain the high pressure throughout the melting operation. This pressure technique can be utilized in conjunction with conventional arc melting or an ordinaryfurnace or the like. Thus, a'cooledmetal crucible can be used to contain the crude sponge, with heat supplied by an arc to the surface of the melting mass. Alternatively, some of the mass can be preliminarily melted by a conventional are, forming a pool of halide salt on top' of titanium metal, and the electrode can be inserted into such halide pool, the latter serving in itself as'a resistance heating element. A particularly useful form of'furnace comprises the externally cooled titanium skull type of apparatus disclosed in my copending application Ser. No. 387,011, filed October 19, 1953, the outer shell of such apparatus being suitably strengthened to Withstand the high pressures and temperatures herein contemplated, and its by-product lowered to a level adapted to more readily effect molten salt removal. Another variation may be to heat a graphite crucible containing the mass by high frequency induction, the entire furnace being enclosed by a cooled metal shell which withstands the high internal pressure. Various other embodiments and modifications of my invention will suggest themselves to those skilled in the art.

The following examples are given simply to illustrate my invention and not in any way to limit its scope:

Example I Sponge titanium as produced by the reaction between titanium tetrachloride and magnesium was charged into a dense graphite crucible 10 inches in diameter and 20 inches in height fitted with a loose cover and inserted within a fused silica tube. The amount of material charged was 20 lbs., it being of about 50% elementary titanium and 50% MgCl The space between the silica tube and the crucible was packed with lampblack. This apparatus was placed Within an externally-cooled metal shell to resist the internal pressure which would be developed, an induction heating coil being connected within-the said shell. Air was evacuated from the shell and hence from the silica tube and crucible, and argon of 99.8% purity was introduced until the absolute pressure within the melting equipment was 150 p. s. i. A high frequency potential of 400 volts, at 9600 cycles per second, was then applied to the induction coil; 17-25 C., the melting point of titanium, was reached in 65' minutes, and the current was then shut off. After cooling, the furnace was vented and opened. The graphite crucible was found to contain an ingot of solid titanium situated beneath a block of solid, white crystalline magnesium chloride. The latter was easily crumbled away from the titanium ingot, and the titaniummetal was found to be of extremely high quality, pure and ductile and easily fabricated.

Example 11 Titanium sponge as produced by the reaction between Sodium and titanium tetrachloride at around 900 C.

was used in this example after a substantial amount ofthe sodium chloride had been drained from the sponge. This sponge, analyzing about 50% sodium chloride, was charged into a copper crucible, which was water-jacketed for cooling. The crucible was then fitted with a gasketed air-tight cover, through the center of which an electrode extended downward to the mass of sponge. Air was evacuated from the chamber via a conduit, also in the cover, and pure argon then introduced therethrough until the internal pressure was 200 p. s. i. a. An arc was struck between the electrode and the sponge, the latter being rapidly melted thereby. When melting was complete, the mass was cooled and removed from the crucible. An ingot of pure, ductile titanium was recovered, on top of which was a stratum of white crystalline sodium chloride which easily flaked or crumbled away.

While described as applied to certain specific adaptations, the invention, it will be understood, is not limited thereto. Thus, while a temperature of at least 1725 C. is mentioned, those ranging from 17301750 C. are preferred for use since they comprise the most usefully-employable. If desired, temperatures of a higher order and ranging up to, say, 1800 C., can also be employed. Similarly, while specific pressures have been mentioned, these are variable and in any given instance will depend upon the nature and vapor pressure of the reducing metal salt present in the titanium sponge material being treated. Hence, while use is contemplated of pressures ranging from 1 to 6 times the vapor pressure of such reducing metalhalide impurity at the melting temperature in general, any pressure which is at least equal to the vapor pressure of such halide impurity and adequate to avoid its distillation from the systemin gaseous form can be used, whereby loss will be avoided of the heat of vaporization of such halide were it allowed to go cit as a gas. Pressures at least equal to the vapor pressure of the salt at the melting temperature, and from about 5 up to, say, 10 atmospheres above such pressure, are, as already indicated, preferred.

It will be apparent from my above description that my process is useful to purify and melt crude titanium sponge wherein any halide by-product is trapped. Hence, the usual reducing metals, the usual titanium halides, and the common reducing methods employed in making titanium metal therefrom are all operable in my invention. It is generally preferred in industrial practice to utilize titanium tetrachloride as the halide, because of its availability and comparative cheapness; similarly, the most common reducing agents are the alkali or alkaline earth metals, such as magnesium, sodium, potassium, lithium and calcium.

The specific gravtiy of molten titanium is about 4.3 while the alkali earth and alkaline earth metal halides are less than /2 of this value. It is thus obvious that the two materials will separate into two layers due to difference in weight and may be readily removed as liquid from the melting zone by obvious methods or they may be allowed to cool after which the solidified materials may be separated by breaking the halide from the titanium ingot.

A particularly important advantage of my invention is that purification and melting of the titanium may be achieved simultaneously in one operation. This short cut naturally lowers cost, increases eflieiency, and reduces the engineering hazards entailed in complex processes. Additionally, it is possible for the first time to effect easy separation of pure titanium from residual contaminants.

Although the invention is particularly adaptable to the separation of titanium from by-product metal halide salt impurity, it is also adaptable to the treatment of other group IV metals, such as zirconium, hafnium and thorium, to efiect their recovery in pure state from their admixture With such impurity, and especially from reducing metal chlorides introduced therein in the reduction of their chlorides.

I claim as my invention:

1. A process for the purification of an impure Fourth Group metal selected from the group consisting of titanium, zirconium, hafnium and thorium to remove a metal halide contaminant therefrom, comprising heating said impure metal under an inert atmosphere in a titanium vessel within a heating zone externally cooled to maintain said vessel in solid state throughout said heating to a temperature above the melting point of said metal and ranging from 1725-1800 C. and under a pressure suflicient to prevent substantial vaporization of said contaminant and ranging from 1 to 6 times the vapor pressure thereof, and recovering the separated, purified metal from the liquefied products formed in the process.

2. A process for the recovery of pure titanium in ingot form which comprises subjecting sponge titanium in admixture With the chloride of the reducing metal selected from the group consisting of alkali and alkaline earth metals employed in obtaining said sponge to treatment at a temperature in excess of about 1725 C. and up to about 1800 C. under an inert atmosphere and at a pressure sufiicient to prevent vaporization of said chloride and to maintain said metal chloride in the liquid state Within a heating zone and in a titanium vessel, externally cooling said zone to maintain said vessel solid throughout said sponge treatment, continuing said treatment until the titanium and said chloride salt form separable, liquid layers, and separating the titanium layer for solidification into an ingot from the metal chloride layer.

3. A method for removing magnesium chloride impurity from titanium metal sponge to recover pure titanium which comprises heating said sponge to temperatures ranging from 1725 C.1800 C. in an atmosphere of argon and under a pressure ranging from 75450 pounds per square inch absolute within a titanium vessel in a heating zone externally cooled to maintain said vessel in solid state, continuing said heating until the titanium and magnesium chloride present in said sponge form into separable liquid layers, cooling the resulting stratified liquid mass to effect solidification thereof, and then separating and recovering the solidified titanium from the magnesium chloride solid.

4. A method for removing magnesium chloride impurity from titanium metal sponge to recover the titanium in pure form which comprises heating said sponge to temperatures ranging from 1730 C.1750 C. in an atmosphere of argon and under a pressure ranging from -210 pounds per square inch absolute within a titanium vessel in the heating zone, externally cooling said zone to maintain said vessel in solid state throughout the purification operation, continuing said heating until the titanium and magnesium chloride present in said sponge form separated, stratified liquid layers, cooling the stratified layers to effect solidification thereof, and then recovering the solidified pure titanium from the solid magnesium chloride solid.

References Cited in the file of this patent UNITED STATES PATENTS 1,568,685 Moore Jan. 5, 1926 1,648,954 Marden Nov. 15, 1927 1,814,719 Marden et a1 July 14, 1931 1,854,234 Stanley Apr. 19, 1932 2,121,084 Kruh June 21, 1938 2,205,854 Kroll June 25, 1940 2,427,338 Alexander Sept. 16, 1947 2,548,897 Kroll Apr. 17, 1951 2,556,763 Maddex June 12, 1951 2,564,337 Maddex Aug. 14, 1951 2,678,267 Saunders May 11, 1954 2,703,752 Glasser et a1 Mar. 8, 1955 OTHER REFERENCES Handbook of Nonferrous Metallury, Recovery of Metals, by Liddell. Pub. 1945 by McGraw-Hill Book Co., Inc., New York, N. Y. Pages 597598.

Metal Industry, October 8, 1948, pp. 283-286. 

1. A PROCESS FOR THE PURIFICATION OF AN IMPURE FOURTH GROUP METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, HAFNIUM AND THORIUM TO REMOVE A METAL HALIDE CONTAMINANT THEREFROM, COMPRISING HEATING SAID IMPURE METAL UNDER ZONE EXTERNALLY COOLED TO MAINVESSEL WITHIN A HEATING ZONE EXTERNALLY COOLED TO MAINTAIN SAID VESSEL IN SOLID STATE THROUGHOUT SAID HEATING TO A TEMPERATURE ABOVE THE MELTING POINT OF SAID METAL AND RANGING FROM 1725-1800*C. AND UNDER A PRESSURE SUFFICIENT TO PREVENT SUBSTANTIAL VAPORIZATION OF SAID CONTAMINANT AND RANGING FROM 1 TO 6 TIMES THE VAPOR PRESSURE THEREOF, ASND RECOVERING THE SEPARATED, PURIFIED METAL FROM THE LIQUEFIED PRODUCTS IN THE PROCESS. 